<!DOCTYPE html>
<html>
<head>
	<title></title>
</head>
<body>
<ul id='ul1'>
	<li></li>
	<li></li>
	<li id='li1'>aa</li>
	<li></li>
</ul>
<script type="text/javascript">
	window.onload=function(){
		var oLi = document.getElementById('li1')

		oLi.previousSibling.style.background='red'
	}
</script>
</body>
</html>